The following invention relates to gaseous working fluids which are compressed, heated and expanded to generate power. More particularly, this invention relates to working fluids for a semi-closed Brayton cycle power system which includes a gas turbine operating on the working fluid which is partially recirculated, the system eliminating emission of pollutants and emitting carbon dioxide in an easily separated and recoverable form.
Gas turbine power systems have become popular systems for meeting modern society""s power needs. Not only do gas turbines provide thrust for most large aircraft, but they also have been adapted for use in generation of electricity in stationary power plants.
Gas turbines operate on the Brayton cycle and have a working fluid, typically air, which remains gaseous throughout the cycle. While the Brayton cycle can theoretically be closed so that the working fluid recirculates, the vast majority of operational gas turbine power plants operate as an open Brayton cycle. In the open Brayton cycle commonly found in commercial stationary power plants, air is drawn into a compressor where its pressure and temperature increase. The temperature of the air is then further increased by combusting a fuel, most often natural gas, in the air to produce a working fluid including air (minus the oxygen which reacts with the fuel) and the products of combustion of the oxygen and the fuel (typically carbon dioxide and steam). This high temperature high pressure working fluid is fed into a turbine where the working fluid is expanded and its temperature and pressure decreased. The turbine drives the compressor and typically additionally drives a generator for the generation of electric power. The working fluid is exhausted from the turbine in a simple open Brayton cycle.
Most operational stationary gas turbine power systems include a simple open Brayton cycle only as one part of a combined cycle. Specifically, because the working fluid still has a relatively high temperature when exiting the turbine, this heat can be used, such as to generate steam in a heat recovery steam generator before finally being exhausted. The steam heated within the heat recovery steam generator can then be utilized to drive a steam turbine, such as that found in any typical closed Rankine cycle steam power plant. When operated as a combined cycle, the open Brayton cycle gas turbine and closed Rankine steam turbine combine to most efficiently extract power from the fuel combusted within the gas turbine (in some systems over 50% thermal efficiency).
While significant advances in compressor and turbine designs have greatly increased the efficiency with which the gas turbine operates and have increased the temperature at which the gas turbine can operate, the gas turbine has certain drawbacks. One drawback of the open Brayton cycle gas turbine is that the exhaust includes oxides of nitrogen (NOx). NOx is a pollutant which can only be emitted in compliance with strict environmental regulations within the United States. Also, open Brayton cycle gas turbines emit carbon dioxide (CO2) into the atmosphere. While emission of CO2 is not currently regulated by the United States government, mounting scientific evidence has connected the emission of CO2 with global warming and other negative atmospheric effects. Numerous proposals are being evaluated for regulation of the emission of CO2. Accordingly, a need exists for a way to eliminate the emission of NOx, CO2 and other pollutants from gas turbines.
Techniques do exist for reduction of the emission of NOx and the elimination of CO2 from open Brayton cycle gas turbine exhaust. The exhaust can be scrubbed of a significant portion of the NOx by various different processes applied to the exhaust to either convert (i.e. using ammonia) or separate the NOx from the exhaust. Such xe2x80x9cscrubbersxe2x80x9d not only decrease the efficiency of the operation of the gas turbine, they are costly and also fail to remove all of the NOx from the exhaust.
Large quantities of CO2 are produced within the open Brayton cycle gas turbine as one of the major products of combustion of the natural gas in air. This CO2 is exhausted in gas form mixed with the large amount of nitrogen in the air which passes through the gas turbine. If removal of the CO2 is desired, the CO2 must first be separated from the nitrogen and other gaseous constituents of the working fluid (i.e. by chemical and/or physical absorption of the CO2, or endothermic stripping processes for separating CO2 from the exhaust gases). The CO2 can then be used in industrial processes or can be eliminated, such as by pressurization and sequestration in underground or deep sea locations. While such CO2 sequestration is a known technique, significant energy is utilized in separating the CO2 from the nitrogen, and hence the efficiency of the open Brayton cycle gas turbine is significantly decreased when CO2 separation is required. Accordingly, a need exists for more efficient separation of the CO2 from other portions of the working fluid so that the efficiency of the gas turbine is not radically diminished.
Closed Brayton cycle gas turbines have been developed for certain specific applications, such as for gas turbines which operate in nuclear power plants. In the closed Brayton cycle a working fluid is provided (typically Helium in nuclear power plants) which remains separate from the heat source and which is recirculated from the turbine exhaust back to the compressor without ever being exhausted. The compressor typically needs modification when the gas being compressed is changed. Because the working fluid is not exhausted, it is not a source of atmospheric pollution. The heat source which heats the working fluid can be nuclear, solar, geothermal or some other form of renewable non-polluting heat source so that atmospheric emissions are avoided.
However, if combustion of a hydrocarbon fuel with air is utilized to heat the working fluid between the compressor and the turbine, the closed Brayton cycle gas turbine will still have an exhaust which includes CO2 and NOx. While renewable non-polluting heat sources such as nuclear, solar and geothermal are effective, they suffer from drawbacks which have limited their ability to be fully competitive with hydrocarbon fuel combustion powered gas turbine systems. Other closed Brayton cycle or partially closed Brayton cycle gas turbine power systems have been proposed which utilize a mixture Of CO2 and oxygen as the combustion medium. For instance, see U.S. Pat. No. 5,724,805 to Golomb. While such CO2 closed Brayton cycle gas turbine systems do keep nitrogen out of the combustor and so do not produce NOx, the high density of CO2 makes it ill suited for use within a compressor which has been designed for compression of air. Accordingly, a need exists for a Brayton cycle gas turbine which heats the working fluid by combustion of a hydrocarbon fuel and which avoids emission of pollutants into the environment.
Another known technique for modifying prior art open Brayton cycle gas turbines is to inject steam into the combustor upstream of the turbine. When steam is injected into the combustor, the power output and the efficiency of the open Brayton cycle can be enhanced. Various different prior art steam injection open Brayton cycles are disclosed by Wilson and Korakianitis in The Design of High-Efficiency Turbo Machinery and Gas Turbines, Second Edition, 1998, Prentice-Hall, Inc. For instance, Wilson and Korakianitis cite one study by the General Electric Corporation that their LM5000 gas turbine, when fitted with steam injection and intercooling will experience a power increase from 34 MW to 110 MW and an efficiency improvement from 37% to 55%, compared to a simple Brayton cycle gas turbine power system with no associated Rankine cycle. Such steam injection open Brayton cycles typically do not operate as part of a combined cycle, but rather utilize the heat recovery steam generator to turn feed water into steam for injection upstream of the turbine. Hence, by steam injection, high efficiencies and high power outputs are provided without requiring a separate steam turbine and condenser as required for a combined cycle.
Steam injection open Brayton cycles also suffer from numerous drawbacks. Such cycles require feed water purification to keep the machinery in good working order. Water purification costs hence impede the desirability of prior art steam injection open Brayton cycles. Also, prior art steam injection open Brayton cycles still produce NOx, carbon dioxide and other pollutants which are emitted into the atmosphere as with the non-steam injection open Brayton cycle gas turbine power systems described above.
The prevalence of open Brayton cycle gas turbines and particularly combined cycle gas turbine power systems throughout the world which are emitting large amounts of NOx and CO2 into the environment makes desirable the provision of a method and apparatus for retrofitting open Brayton cycle gas turbines in a manner which does not interfere with the existing equipment but which eliminates emission of nitrogen oxides, CO2 and other pollutants into the atmosphere, so that capital costs associated with such retrofits can be minimized. Such retrofits would additionally benefit from the use of a working fluid which matches the performance characteristics of working fluids in known prior art open Brayton cycle systems so that optimum performance of the system components can be maintained.
The needs for pollutant emission elimination and gas turbine efficiency preservation identified above are met by the semi-closed Brayton cycle gas turbine power system of this invention and the associated working fluids employed by this system. This power system utilizes all of the major components of an open Brayton cycle gas turbine power system and optionally also the major components of a combined cycle power system. Additional equipment is added to recirculate at least a portion of the working fluid exiting the turbine and to cool the exhaust if necessary so that it passes back to the compressor forming a semi-closed Brayton cycle.
Specifically, the semi-closed Brayton cycle power system of this invention includes a divider coupled to the turbine outlet of an otherwise known prior art open Brayton cycle gas turbine. The divider splits the exhaust flow of the working fluid exiting the turbine output. A portion of the divider leads to a return duct which directs a major portion (approximately 85%) of the turbine exhaust back to the compressor inlet. The other portion of the divider leads to a separation duct which leads to a condenser having a condensate outlet and a gas outlet.
An oxygen duct directs oxygen into the return duct so that oxygen is added to the portion of the turbine exhaust which passes from the divider into the return duct. The oxygen entering the return duct is mixed with the exhaust therein so that the compressor inlet receives a mixture of the turbine exhaust and the oxygen.
The compressor gas mixture typically includes three gases which are mixed together. These gases include oxygen, steam (water vapor) and CO2. The percentage of the gas mixture which each one of these constituents provides can vary. A preferred simple constitution of the gas mixture can be 13% wt oxygen, 39% wt water and 48% Wt CO2. These constituent percentages can vary somewhat. Preferably, the constituents which form the compressor gas mixture are present at a ratio which is selected so that the gas mixture mimics the properties of air, which is itself a mixture of gases. At the preferred constituent percentages identified above, the various quantifiable physical properties of air (i.e. gas constant, specific heat, density, etc.) are closely matched. Hence, the compressor compresses the gas mixture in the same manner that it compresses air without operating outside of its design limits.
The compressed gas mixture enters the combustor where natural gas or pure methane is combusted with the oxygen in the gas mixture. The methane combusts with the oxygen in the gas mixture just as it would with the oxygen in air. If sufficient methane is supplied to consume all of the oxygen in the gas mixture (a stoichiometric mixture ratio), the working fluid exiting the outlet port of the combustor is entirely CO2 and steam. These gases have two sources, CO2 and steam from the gas mixture entering the combustor and CO2 and steam generated as products of combustion of the oxygen and the methane.
This working fluid passes through the turbine and exits the turbine output as the exhaust. Because the exhaust is entirely CO2 and steam, no NOx is present and no NOxelimination equipment need be utilized. If the semi-closed cycle is optionally acting as part of a combined cycle, the exhaust passes through a heat recovery steam generator where it gives up heat to the steam in the xe2x80x9cbottomingxe2x80x9d Rankine cycle. The exhaust then exits the heat recovery steam generator and enters the divider. A portion of the exhaust is directed to the return duct where it is directed back to the compressor. This exhaust, when mixed with the oxygen from the oxygen duct returns to the appropriate proportions necessary to constitute the gas mixture described in detail above. The gas mixture then again passes through the semi-closed Brayton cycle as described above.
A portion of the exhaust entering the divider is diverted into the separation duct. This exhaust enters a condenser. Because the exhaust is entirely CO2 and water, and because water condenses at a much higher temperature than CO2, the condenser can very effectively and efficiently condense the water while the CO2 remains gaseous. A condensate outlet removes the water portion of the exhaust. The water is pure and can be utilized as desired without contamination of the environment.
The condenser gas outlet removes CO2 from the condenser. This CO2 is essentially pure. Hence, the high energy process of removing CO2 from nitrogen which would be necessary to separate CO2 from exhausts of prior art open Brayton cycle gas turbines is avoided. The CO2 can be sold as an industrial gas, utilized beneficially or can be compressed and sequestered in an underground sequestration site, deep ocean location or any other suitable terrestrial formation.
Because the compressor gas mixture and other working fluids have properties which mimic those of air the major components of the open Brayton cycle gas turbine can be left unmodified and the remaining portions of the semi-closed Brayton cycle of this invention can be added so that an open Brayton cycle gas turbine power system can be retrofitted and modified into a non-polluting power plant. Such a retrofit can occur both for a simple open Brayton cycle gas turbine power system with addition of an appropriate turbine exhaust cooling heat exchanger and for a combined cycle power system.
The semi-closed Brayton cycle gas turbine power system of this invention can be adapted to utilize steam injection upstream of the turbine to provide the semi-closed Brayton cycle with the enhanced efficiency and power output benefits of steam injection detailed above. Because the semi-closed power system generates purified water, this generated water source is used and a separate purified water source is not required. Specifically, in the semi-closed cycle with steam injection a partial condenser is located within the return duct which condenses some of the steam out of the exhaust. The water produced by the condensation of some of the steam in the exhaust is routed through the heat recovery steam generator where it is converted back into steam.
This pure steam is then injected upstream of the turbine. For instance, the steam can be injected with the fuel, injected with the oxidizer from the compressor, or injected separately into the combustor or between the combustor and the turbine. While not preferred, excess water exiting the condenser downstream from the separation duct could similarly be utilized for steam injection.
When the steam is produced from water extracted from a partial condenser in the return duct, the ratio of steam to carbon dioxide within the working fluid passing through the return duct is decreased. As a result, the compressor can compress more oxygen and less steam with the same amount of work. With more oxygen passing through the compressor, more fuel can be combusted in the combustor and the power output of the semi-closed cycle is increased. Also, efficiency of the cycle is increased. While steam injection is typically utilized as a replacement for the xe2x80x9cbottomingxe2x80x9d Rankine cycle of the semi-closed combined cycle, steam injection could be utilized within a semi-closed combined cycle power system with the heat recovery steam generator generating steam for injection into the combustor upstream of the gas turbine and also generating steam for use within the bottoming Rankine cycle.
Accordingly, a primary object of the present invention is to provide a Brayton cycle gas turbine power system which does not emit NOx or other pollutants, and which efficiently collects CO2 for beneficial use or elimination.
Another object of the present invention is to provide a Brayton cycle gas turbine power system which recirculates a portion of the turbine exhaust for input into the compressor of the power system.
Another object of the present invention is to provide a process for modifying an open Brayton cycle gas turbine to function as a semi-closed Brayton cycle gas turbine power system which substantially eliminates emission of pollutants.
Another object of the present invention is to provide substantially nitrogen free air substitute working fluids which can operate within a Brayton cycle gas turbine without significantly altering the performance of the gas turbine and eliminate pollutant emissions from the gas turbine.
Another object of the present invention is to provide a power system which can efficiently generate power from the combustion of hydrocarbon fuels without emission of pollutants.
Another object of the present invention is to provide a semi-closed gas turbine power system with steam injection to enhance the power output and, efficiency of the power system.
In addition to the above objects, various other objects of this invention will be apparent from a careful reading of this specification including the detailed description contained herein below.